Webb & Hubble find massive star clusters emerge faster

Astronomers have long known that understanding how star clusters come to be is key to unlocking other secrets of galactic evolution. Stars form in clusters, created when clouds of gas collapse under gravity. As more and more stars are born in a collapsing cloud, strong stellar winds, harsh ultraviolet radiation and the supernova explosions of massive stars eventually disperse the cloud, ending star formation before all the gas is used up. Once the cloud of gas a star cluster was born in is gone, its light can bear down on other star-forming regions in the galaxy, too. This process is called stellar feedback, and it means that most of the gas in a galaxy never gets used for star formation. Researching how star clusters develop, then, can answer questions about star formation at a galactic scale.

Studies of the closest star-forming regions, in the Milky Way galaxy and the dwarf galaxies that orbit it, allow us to dissect star clusters in the smallest details, but our position in the disc of our galaxy means only a few such regions are visible to us. By observing nearby galaxies, astronomers can survey thousands of star-forming regions and characterise entire populations of star clusters at many stages of evolution – a feat made possible with the launch of space telescopes, most prominently the NASA/ESA Hubble Space Telescope. Both kinds of investigation are necessary to truly understand how star formation takes place in galaxies.

The continuous development of infrared astronomy has allowed us to pull back the gaseous curtains that still hide the youngest star clusters and learn about the earliest stages of their development, but some subjects still puzzle researchers. For instance: when a star cluster forms, what determines how long it takes to disperse its natal cloud and begin radiating ultraviolet light out into the galaxy?

Now, the state of the art has been further developed with both Hubble and Webb working together to provide a broad-spectrum view of thousands of young star clusters. An international team of astronomers has pored over images of four nearby galaxies – Messier 51, Messier 83, NGC 4449, and NGC 628 – from the FEAST observing programme (#1783), trying to solve this mystery. Their results, published today in Nature Astronomy, show that it is the most massive star clusters that clear away their gaseous shroud the fastest, and begin lighting their galaxy the earliest.

The team identified nearly 9000 star clusters in the four galaxies in different evolutionary stages: young clusters just starting to emerge from their natal clouds of gas, clusters that had partially dispersed the gas (both from Webb images), and fully unobstructed clusters visible in optical light (found in Hubble images). With Webb’s ability to peer inside the gas clouds, they were able to then estimate the mass and age of each cluster from its light spectrum. The most massive clusters had fully emerged and dispersed the clouds of gas after around five million years, while less massive clusters were between seven and eight million years old when they emerged from their nurseries.

Answering this open question of which star clusters clear away their birth clouds the fastest advances our understanding of galaxy formation. “Simulations of star formation and stellar feedback have struggled to reproduce how star clusters form and emerge from their natal clouds. These results give us important new constraints on that process,” explained Angela Adamo of Stockholm University and the Oskar Klein Centre in Sweden, a lead author on the study and PI of the FEAST programme.

Massive star clusters with their abundances of hot stars naturally emit most of the ultraviolet light in galaxies, but this work confirms that they also get a head start on producing stellar feedback over lighter clusters. Knowing where and when this stellar feedback is strongest throughout the lifetime of a galaxy allows astronomers to better predict how star-forming fuel is pushed around the galaxy and therefore how stars, and star clusters, are likely to form.

Our theories of how planets form are also impacted by this research. The faster gas is cleared away within a star cluster, the earlier protoplanetary discs around stars are exposed to harsh ultraviolet radiation from other stars, and the less opportunity they have to attract further gas from the nebula. This reduces the opportunities they have to grow dust and create planets.

“This work brings together researchers simulating star formation and those working with observations, as well as groups researching planet formation,” said Alex Pedrini, lead author, also of Stockholm University and the Oskar Klein Centre in Sweden. “Using Webb, we can look into the cradles of star clusters and connect planet formation to the cycle of star formation and stellar feedback.”

More information

Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).

Science paper

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